Spin dependent tunneling in junctions involving normal and superconducting CDW metals

1
Spin dependent tunneling in junctions involving
normal and superconducting CDW metals

• A.M. Gabovich and A.I. Voitenko (Institute of
  Physics, Kyiv, Ukraine)
• T. Ekino (Hiroshima University, Japan)
• Mai Suan Li and H. Szymczak (Institute of
  Physics, Warsaw, Poland)
• M. Pekala (Warsaw University, Poland)

2
Introduction

• Electronics vs. Spintronics
• Ferromagnets
• Magnetization is linked to the difference
  between spin sub-band populations in the
  conduction band
• Objective To estimate the polarization of the
  tunnel current

3
Starting points of Tedrow and Meservey (1973)

• Tunnel conductances G(V) for metal/gapped
  material junction at temperature T?0 the Fermi
  distribution of metal electrons serves as a probe
  of the electron density of states (DOS) of the
  gapped material electrode
• Factors
• in the integrand of G(V) are caused by metal
  electrons

4
TMs original idea for FMBCS junction

• If the gapped material BCS superconductor, its
  peak-possessing DOS may also serve as a probe of
  the metal DOS in the vicinity of the Fermi
  surface (FS) !
• Warning absence of an electron spin-flipping
  while tunneling
• Problem
• To segregate the spin-polarized components of
  the tunnel current

5
Splitting of spin sub-bands in the BCS
superconductor

• Solution
• Spin sub-bands in a BCS s-wave superconductor can
  be split in an external magnetic filed, ?B is
  the effective Bohr magneton
• To apply H
• Requirement
• Availability of a gapped FS section on one side
  and a non-gapped FS section on the other side of
  the junction

6
New problems and their solutions

• Meissner effect
• Thin films.
• Temperature smearing
• Use as low T as possible.
• In any case, T lt Tc.
• Spin-orbit interaction Z4
• Use constituting elements as light as possible.
• The effect was measured for Al
• Z 13, ?0.4 meV, Tc 1.19 K
• Counter-electrodes Fe, Ni, Co.

7
Our idea To use CDW metals

• CDW metal
• FS comprises both gapped (d) and non-gapped (nd)
  sections.
• The DOS structure
• at the d-sections is similar to that of BCS
  superconductor (the dielectric order parameter
  S),
• at the nd-section to that of ordinary metal (no
  gap).

• Advantages
• No Meissner effect
• Less stringent requirements to sample geometry
• Bigger range of the dielectric gaps S critical
  temperatures Td is in the range 1 K ?1000 K
• Spin-splitting is observable in the symmetrical"
  (CDWM/CDWM') setup
• Possibility to use the effect in studying CDWMs
  themselves.
• For example 2H-NbSe2
• ZNb 41 (ZSe 34), S 34 meV,
• Td 33.5 K

8
Greens function method of the tunnel current
calculation

• FMCDWM junction

9
FMCDWM junction

• Drastic distinctions from the FMBCS case
• peaks on one CVC branch and cusps on the other
  one, h ?BH/?0, ?0 ?(T0)
• Strong dependence on the parameter µ

10
Sensitivity to the parameter P and to the sign of
S
11
CVCs for the FM Isuperconducting CDWM junction
- superconducting gap for T 0
in the absence of CDWs
12
CDWM'CDWM setup

• (

• Symmetrical CDWMCDWM junction
• Distinction from FMBCS case
• Different disposition ( - - ) of spin-polarized
  peaks
• BCS (- - )

no effect in BCS'BCS setup) Energy scheme,
processes with spin splitting - - - - -
without spin splitting
13
Sensitivity to

• gapping level µ

• temperature

14
CVCs for the CDWM ICDWM junction

• CDWMs are normal
• The phase of the left electrode equals to zero

15
Conclusions

• CDW metals (CDW superconductors)
• Can be used in tunnel experiments to detect spin
  splitting
• Possess advantages over BCS superconductors
• no Meissner effect
• bigger range of gap amplitudes
• Can be observed in symmetrical junctions, since
  there are both degenerate and non-degenerate FS
  sections
• are perspective objects for investigation in
  spintronics